The 6 player came because of Kickstarter.
It is in all games.
The word of caution--if you have players prone to think out all possibilities, the 5-6 player games can drag. It's much better if people are familiar before doing it.
Very True! We had a 6-player game where all but one of us had played it before and due to much bad dice rolling (mostly - it was a true horror show) it took almost 4 hours, with almost all of it spent on exploration. I think part of speeding things up might be to convince people to actually look at their current cards (especially the bottom portion) BEFORE their turn. Drawing two cards at the beginning of the turn leads to 1+ minutes of study before the turn actually begins, then many re-rolls later (Parrot - card - card...) they are done.